Most computer systems use a form of raster image to output image data to a visual output device such as a cathode ray tube (CRT) monitor, liquid crystal display (LCD), or a printer such as a color laser copier. A raster image is represented as a series of scan lines of pixels, wherein each pixel is accessed in a sequential manner, and each scan line is processed. An area to which image data may be output sequentially by the visual output device is referred to as a page. The image data of a page provides its complete visual representation.
Interpreters of page description languages typically process images in one or both of two modes: band mode and frame mode. In band mode, the entire page description is read, and a list of every imaging operation, called a display list, is built including all the data needed to image the page. Since all of the data also may include very large scanned images, a display list can grow to be very large. Scanned images require that the display list be temporality stored on secondary storage, with a performance reducing step required both to read and write. A fundamental advantage, however, is that the output frame is only traversed once, which permits lower speed devices to be connected with a minimum of buffering.
Frame mode on the other hand, does not include constructing a display list. Instead, a full image exists, and as each imaging operation is encountered, it is immediately processed to update the fill page image. Scanned images are also processed immediately, eliminating the problem of temporarily storing them. Thus, whereas band mode traverses the output page image in an orderly fashion, sweeping from one edge to the opposite edge, frame mode accesses the output page randomly.
Both of these however generally require a buffer memory which can hold the entire visual representation of a page. This memory is referred to as a frame buffer. Because of the large memory requirements for representation of a page, particularly with color laser copiers and color images, it is desirable to reduce the buffer memory requirements, for example by using some form of compression.
One compression method is shown in U.S. Pat. No. 5,506,944 to Ronald S. Gentile. This reference shows a system in which an image is divided into several regions. A page description, describing objects to be placed on the page, is ordered so that commands for each region are processed together. The regions are processed sequentially. A single memory element capable of storing the image data of an entire region in an uncompressed form is provided. After each region is processed, its corresponding image data is stored in a stack (a last-in, first-out buffer) in compressed format. When a page has been completely processed, this compressed image data is then read out of the stack to be decompressed and output to a printer.
A problem with this system however is that it requires a page description to be completely processed or ordered before processing of the frames can begin. It does not display operations to be processed in an arbitrary order. Additionally, it may suffer from the same problems as band mode operation when the display list becomes too large.